Various analytical procedures and devices are commonly employed in flow-through assays to determine the presence and/or concentration of analytes that may be present in a test sample. For instance, immunoassays utilize mechanisms of the immune systems, wherein antibodies are produced in response to the presence of antigens that are pathogenic or foreign to the organisms. These antibodies and antigens, i.e., immunoreactants, are capable of binding with one another, thereby causing a highly specific reaction mechanism that may be used to determine the presence or concentration of that particular antigen in a biological sample.
There are several well-known immunoassay methods that use immunoreactants labeled with a detectable component so that the analyte may be detected analytically. For example, “sandwich-type” assay formats typically involve mixing the test sample with detection probes conjugated with a specific binding member (e.g., antibody) for the analyte to form complexes between the analyte and the conjugated probes. These complexes are then allowed to contact a receptive material (e.g., antibodies) immobilized within the detection zone. Binding occurs between the analyte/probe conjugate complexes and the immobilized receptive material, thereby localizing “sandwich” complexes that are detectable to indicate the presence of the analyte. This technique may be used to obtain quantitative or semi-quantitative results. Some examples of such sandwich-type assays are described in. by U.S. Pat. No. 4,168,146 to Grubb, et al. and U.S. Pat. No. 4,366,241 to Tom, et al. An alternative technique is the “competitive-type” assay. In a competitive assay, the labeled probe is generally conjugated with a molecule that is identical to, or an analog of, the analyte. Thus, the labeled probe competes with the analyte of interest for the available receptive material. Competitive assays are typically used for detection of analytes such as haptens, each hapten being monovalent and capable of binding only one antibody molecule. Examples of competitive immunoassay devices are described in U.S. Pat. No. 4,235,601 to Deutsch, et al., U.S. Pat. No. 4,442,204 to Liotta, and U.S. Pat. No. 5,208,535 to Buechler, et al.
Despite the benefits achieved, conventional lateral flow assays still exhibit significant problems. For instance, some assays encounter significant inaccuracies when exposed to relatively high analyte concentrations. When the analyte is present at high concentrations, a substantial portion of the analyte in the test sample may be left in excess and therefore not form complexes with the conjugated probes. Thus, upon reaching the detection zone, the uncomplexed analyte competes with the complexed analyte for binding sites. Because the uncomplexed analyte is not labeled with a probe, it cannot be detected. Consequently, if a significant number of the binding sites become occupied by the uncomplexed analyte, the assay may exhibit a “false negative.” This problem is commonly referred to as the “hook effect” or “prozone”.
Besides encountering problems at high analyte concentrations, the calibration systems employed by conventional assays are often unreliable. For example, some assays use external calibration systems in which a curve is obtained from standard samples containing a series of known amounts of analyte. The test results may then be compared with the standard curve to extract the presence and/or amount of the analyte in the sample. The external calibration method, however, is often subject to interference from environmental and batch-to-batch variations, and is thus unreliable. Some internal calibration systems have thus been developed to overcome these problems. Unfortunately, many internal calibration techniques are not readily incorporated into lateral flow devices, which involve heterogeneous separation of the analyte using chromatographic methods.
As such, a need still exists for an integrated diagnostic test kit that is capable of accurately determining the presence or quantity of an analyte over a broad range of possible concentrations.